1. Field of the Invention
This invention relates to moving systems for carrying out displacement of the optical head of a videodisk reader-recorder in the radial direction along the disk in order to follow the information-carrying track. In addition, the moving systems serve to maintain focusing of the objective with which said head is provided as well as to carry out fine following of the track. The invention further relates to the videodisk reader-recorders which are equipped with a moving system of this type as well as to readers and recorders alone.
2. Description of the Prior Art
A well-known design of moving system consists of a carriage which is displaced by means of a lead-screw. The reading head placed on said carriage accordingly comprises an objective placed within a magnetic pot which is similar to the motor of a loudspeaker and serves to maintain focusing of said objective on the disk. Fine following of the track is performed by means of a reflecting mirror mounted on the frame of a galvanometer.
In French patent Application No 78 34 649 filed by the present Applicant on Dec. 8th, 1978 under the title: "A device providing access to a track carried by an optically-readable recordable medium, and an optical system comprising a device of this type", and corresponding to U.S. Pat. No. 4,321,701, there was described a moving system formed essentially by a flat coil of very small thickness and very light weight clamped between two glass plates and capable of both vertical and longitudinal displacement within the air-gap of a magnetic circuit. Track-following is performed by means of two vertical wires and the reading elements are formed by a semiconductor laser and a detector diode which are fixed directly on said moving system.
Although the device just mentioned is of very small mass and is therefore conducive to very low inertia forces, mechanical guidance is difficult to achieve and results in couplings between the track-following and focusing functions. Moreover, the use of simple vertical electric wires for track-following makes it possible to obtain only a low force for the motion control which is necessary for said track-following operation in spite of the need for electric currents which have high values and are therefore difficult to utilize in practice.